In packet switching data transmission systems (e.g., optical packet switching systems) the data signal which arrives at a receiver is in a burst mode. The incoming signal, although representative of binary data, is actually an analog signal that must be interpreted as binary data. This is important to note because the signal levels of the data arriving in burst mode may have widely varying values. In other words, the signal level which constitutes a 1 may be substantially different from one packet to another. Likewise, the signal level which constitutes a 0 may be very different from one packet to another. These signal levels may vary by as much as 10 dB (although it should be noted that the variation depends upon the size and configuration of the system).
Because the signal levels can vary in this manner, it is necessary to identify an appropriate slicing level for each packet in order to be able to properly interpret the data bits in the packet. The slicing level is the signal level above which a data signal is considered to be a 1 and below which a data signal is considered to be a 0. Once the slicing level is determined, the analog signal can be interpreted as binary 1's and 0's.
While there are several conventional approaches for identifying slicing levels, these approaches have drawbacks which make them inadequate for use in high-speed optical burst-mode transmission systems. For example, one approach is a sample-and-hold integration approach. Sample-and-hold integration methods require a sample-and-hold pulse that is synchronized with the incoming data. The accuracy of the slicing levels determined by these methods can be directly affected by the accuracy of the synchronization. If, for instance, the sampling window is not in alignment with the data packet, the method will integrate a portion of the packet and the remainder of the packet may comprise noise. Since the noise is unrelated to the data signal levels, it will obviously affect the integration calculation. In an optical burst-mode transmission system, it may not be possible (there may not be enough time) to synchronize the sample-and-hold pulse with the packet.
Another conventional approach for identifying a slicing level is a feed-forward peak detector approach. This approach does not require the synchronization pulse of the sample-and-hold methods, and instead uses a reset pulse that can easily be provided. Implementation of the feed-forward peak detector approach in an optical burst-mode system, however, requires that the photodetector be DC-coupled to the limiting amplifier which serves as the decision circuit. This prevents the use of a trans-impedance amplifier with a low-end cut-off frequency.
Yet another conventional approach uses a fast AGC (automatic gain control) loop. The AGC loop uses a self-adjustable pre-amplifier to convert varying levels of incoming data to a stable signal level. The stabilized-level signal can then be properly interpreted by a subsequent digitizing circuit. AGC loops, however, are generally relatively slow and consequently cannot be used in a system with a bit rate greater than 10 Gbps.